// 计算文件的eTag，参数为buffer或者readableStream或者文件路径
export function getEtag(buffer, callback) {
  // 判断传入的参数是buffer还是stream还是filepath
  var mode = 'buffer';

  if (typeof buffer === 'string') {
    buffer = require('fs').createReadStream(buffer);
    mode = 'stream';
  } else if (buffer instanceof require('stream')) {
    mode = 'stream';
  }

  // sha1算法
  var sha1 = function (content) {
    var crypto = require('crypto');
    var sha1 = crypto.createHash('sha1');
    sha1.update(content);
    return sha1.digest();
  };

  // 以4M为单位分割
  var blockSize = 4 * 1024 * 1024;
  var sha1String = [];
  var prefix = 0x16;
  var blockCount = 0;

  switch (mode) {
    case 'buffer':
      var bufferSize = buffer.length;
      blockCount = Math.ceil(bufferSize / blockSize);

      for (var i = 0; i < blockCount; i++) {
        sha1String.push(sha1(buffer.slice(i * blockSize, (i + 1) * blockSize)));
      }
      process.nextTick(function () {
        callback(calcEtag());
      });
      break;
    case 'stream':
      var stream = buffer;
      stream.on('readable', function () {
        var chunk;
        while ((chunk = stream.read(blockSize))) {
          sha1String.push(sha1(chunk));
          blockCount++;
        }
      });
      stream.on('end', function () {
        callback(calcEtag());
      });
      break;
  }

  function calcEtag() {
    if (!sha1String.length) {
      return 'Fto5o-5ea0sNMlW_75VgGJCv2AcJ';
    }
    var sha1Buffer = Buffer.concat(sha1String, blockCount * 20);

    // 如果大于4M，则对各个块的sha1结果再次sha1
    if (blockCount > 1) {
      prefix = 0x96;
      sha1Buffer = sha1(sha1Buffer);
    }

    sha1Buffer = Buffer.concat(
      [Buffer.from([prefix]), sha1Buffer],
      sha1Buffer.length + 1,
    );

    return sha1Buffer
      .toString('base64')
      .replace(/\//g, '_')
      .replace(/\+/g, '-');
  }
}
